In these years, photonic networks for a large capacity communication are building for practicing the next generation network (NGN) which needs an optical transmission system capable of fast transmission. For the fast optical transmission, it will be necessary to adopt a modulation scheme which is able to convert effectively electrical signals to optical signals. The differential phase shift-keying scheme is receiving attention as a superior sensitivity modulation scheme.
Referring to FIG. 7, a conventional optical transmission system using a phase modulation scheme is explained. FIG. 7 illustrates schematically a configuration of the conventional optical transmission system 1 with a phase modulation scheme. The conventional optical transmission system 1 includes an optical transmitter 2 and an optical receiver 3. The optical transmitter 2 includes an optical phase modulator 21 which performs a conversion from an electric signal to an optical signal and a phase modulation of the optical signal, and transmits the phase-modulated optical signal to the optical receiver. The optical signal which is phase-modulated by the optical phase modulator 21 is hereinafter referred to as “a DPSK optical signal.”
The optical receiver includes a delay interferometer 31 and an optical receiving circuit 32 which are illustrated in detail in FIG. 8. The delay interferometer 31 compares the DPSK optical signal received from the optical phase modulator 21 with an optical signal which is derived from the DPSK optical signal delayed by one bit to demodulate the DPSK optical signal. The delay interferometer 31 outputs, to the optical receiving circuit 32, a normal in phase and a reverse in phase components of the optical signal demodulated. The normal in phase component and the reverse in phase component are hereinafter referred to as the normal in phase optical signal and the reverse optical phase signal, respectively.
The optical receiving circuit includes a photo-detector 41, which is also referred to as a photo-detector and hereinafter abbreviated as PD, and an amplifier 42. The PD 41 comprises PDs 41a and 41b. The PD 41a converts the normal in phase optical signal to a corresponding electric signal and the PD 41b converts the reverse in phase optical signal to a corresponding electric signal. The PD 41 outputs, to the amplifier 42, a difference value of the normal and the reverse in phase signals. The difference value is amplified by the amplifier 42 and fed to a device or a circuit which is referred to as a data indentifying device and not shown in FIG. 8.
The data-identifying device determines data, which is corresponding to data sent from the optical transmitter 2, based on the received difference value from the amplifier 42. The data-identifying device determines, as example, the data being “1” when the received difference value is larger than the predetermined value and the data being “0” when the received difference value is smaller or equal to than the predetermined value. A conventional optical transmission system using DPSK optical signal is, for example, disclosed in Japanese Laid-open Patent Publication No. 11-4196.